An electromagnetic fuel injector comprises a cylindrical tubular housing body having a central feed channel, which acts as a fuel conduit and terminates with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve has a pin, which is connected rigidly to a movable armature of the electromagnetic actuator, and is moved by the electromagnetic actuator between a closed position and an open position, respectively closing and opening the injection nozzle, in opposition to a spring which keeps the pin in the closed position. The pin terminates with a shutter head, which, in the closed position, is pushed by the spring against a valve seat of the injection valve to prevent fuel outflow. The shutter head is normally housed inside the fuel conduit, and, to move from the closed to the open position of the injection valve, therefore moves in the opposite direction to the fuel feed direction.
Electromagnetic fuel injectors of the above type are cheap and easy to produce and have a good cost-performance ratio. On the other hand, they fail to provide for precision and stability in the fuel injection direction, and are therefore unsuitable for so-called “spray-guided” engines, in which fuel must be injected precisely close to the spark plug. In this type of application, in fact, an error of less than a millimeter in the fuel flow direction may wet the spark plug electrodes and so seriously impair combustion.
To achieve a highly precise, highly stable fuel injection direction, an electromagnetic fuel injector has been proposed, in which the shutter head is truncated-cone-shaped, is located outside the fuel conduit, is pushed by a spring against the valve seat of the injection valve in the opposite direction to the fuel feed direction, and so moves from the closed to the open position in the same direction as the fuel feed direction.
In injectors in which the pin moves into the open position in the same direction as the fuel feed direction, however, the effect of the difference in thermal expansion of the pin and the housing body has been found to be less than negligible. In actual use, the housing body is in direct contact with the cylinder head of the engine, and so reaches an operating temperature of 120–140° C., whereas the pin, being immersed in the fuel flow, reaches operating temperatures of 60–70° C. The difference in operating temperature results in a corresponding difference in the thermal expansion of the pin and the housing body, which, when significant, alters the size of the fuel passage, with obvious effects on fuel injection flow. In fact, for a given injection pressure, the larger the fuel passage, the greater the fuel injection flow. In other words, injectors in which the pin moves into the open position in the same direction as the fuel feed direction fail to ensure highly precise, highly stable fuel injection flow (and, hence, the amount of fuel injected at each injection) on account of the difference in thermal expansion of the pin and the housing body.
To reduce the negative effect of the difference in thermal expansion of the pin and the housing body, it has been proposed to make the pin and the housing body from steel with a low thermal expansion coefficient (typically, INVAR). Using steel with a low thermal expansion coefficient, however, not only fails to solve the problem completely, but also increases the cost of the injector.
To compensate the difference in thermal expansion of the pin and the housing body, it has also been proposed to connect the pin actuator to a hydraulic compensating device for maintaining a constant distance between the pin armature and the valve seat. Using a hydraulic compensating device, however, makes the injector more complicated and more expensive to produce.
US2002079388 discloses a nozzle assembly for fuel injection in an internal combustion engine and comprising a nozzle tip with a hollow interior defining a fuel chamber. The nozzle tip has at least one spray orifice opening to an outer surface on the nozzle tip, and a valve member at least partially disposed within the nozzle tip; the valve member is moveable between a first position in which the valve member contacts an upper valve seat to prevent fluid communication of fuel from the fuel chamber to the at least one spray orifice, and a second outward position in which the valve member contacts a lower valve seat to allow fluid communication of fuel from the fuel chamber to the at least one spray orifice. The valve member is directly electrically actuated, preferably by a solenoid; further, the valve member is biased in the closed position and the valve member is pressure balanced when high pressure fuel in present in the fuel chamber.
GB2349421 discloses a register nozzle having two through-flow cross-sections for the purpose of injecting fuel, in particular heavy oil, in two stages from a pressure chamber into the combustion chamber of an internal combustion engine. The register nozzle has a nozzle holder and a nozzle body in which a nozzle needle which is pretensioned by virtue of a spring can be displaced in an axial manner by means of a nozzle needle stroke stop device; a sleeve is received in an axially displaceable manner on the nozzle needle in the nozzle body and, when acted upon by pressure, is moved against a sleeve stop, which is formed on the nozzle body, the nozzle needle stroke stop device moving into position against said sleeve in the first injection stage, and a stop being formed on the nozzle body, the nozzle needle stroke stop device moving into position against said stop in the second injection stage.